Perseids

Observer's Synopsis

This meteor shower is generally visible between July 23 and August 22.
Maximum occurs during August 12/13 (Solar Longitude=139 deg), with the
radiant located at RA=48 deg, DECL=+57 deg. The hourly rate typically
reaches 80, although some years have been a
s low as 4 and as high as 200. The meteors tend to be very fast,
possess an average magnitude of 2.3 and about 45% leave persistent
trains. The radiant advances by a rate of 1.40 deg/day in RA and 0.25
deg/day in DECL.

History

This is the most famous of all meteor showers. It never fails to
provide an impressive display and, due to its summertime appearance, it
tends to provide the majority of meteors seen by non-astronomy
enthusiasts.

The earliest record of its activity appears in the Chinese annals,
where it is said that in 36 AD "more than 100 meteors flew thither in
the morning." Numerous references appear in Chinese, Japanese and
Korean records throughout the 8th, 9th, 10
th and 11th centuries, but only sporadic references are found between
the 12th and 19th centuries, inclusive. Nevertheless, August has long
had a reputation for an abundance of meteors. The Perseids have been
referred to as the "tears of St. Lawrence
", since meteors seemed to be in abundance during the festival of that
saint on August 10th, but credit for the discovery of the shower's
annual appearance is given to Quételet (Brussels), who, in 1835,
reported that there was a shower occurri
ng in August that emanated from the constellation Perseus.

The first observer to provide an hourly count for this shower was
Eduard Heis (Münster), who found a maximum rate of 160 meteors per
hour in 1839. Observations by Heis and other observers around the world
continued almost annually thereafter, with ma
ximum rates typically falling between 37 and 88 per hour through 1858.
Interestingly, the rates jumped to between 78 and 102 in 1861,
according to estimates by four different observers, and, in 1863, three
observers reported rates of 109 to 215 per hour. Although rates were
still somewhat high in 1864, generally "normal" rates persisted
throughout the remainder of the 19th-century.

Computations of the orbit of the Perseids between 1864 and 1866 by
Giovanni Virginio Schiaparelli (1835-1910) revealed a very strong
resemblance to periodic comet Swift-Tuttle (1862 III). This was the
first time a meteor shower had been positively identif
ied with a comet and it seems safe to speculate that the high Perseid
rates of 1861-1863 were directly due to the appearance of Swift-Tuttle,
which has a period of about 120 years. Multiple returns of the comet
would be responsible for the distribution of the meteors throughout the
orbit, but meteors should be denser in the region closest to the comet,
so that meteor activity should increase when the comet is near
perihelion (as has been demonstrated by the June Boötids,
Draconids and Leonids).

As the 20th-century began, the maximum annual hourly rates of the
Perseids seemed to be declining. Although rates were above Denning's
derived average rate of 50 per hour during five years between 1901 and
1910, the observed rate in 1911 was only 4 and fo
r 1912 it was 12. Denning wondered whether the shower was declining,
but hourly rates seemed to return to "normal" in the years that
followed. Quite unexpectedly the shower suddenly exploded in 1920, when
rates were estimated to be as high as 20
0 per hour. This was extremely unusual as it came at a time when the
parent comet was nearing aphelion! Although a few weaker-than-normal
years occurred during the 1920's, the Perseids regained their
consistency thereafter, and, except for abnormally high rates of 160
and 189 during 1931 and 1945, respectively, nothing unusual was
observed up through 1960.

During 1973, Brian G. Marsden predicted Comet Swift-Tuttle would
arrive at perihelion on September 16.9, 1981 (+/-1.0 years). This
immediately generated excitement among meteor observers as the
potential for enhanced activity unfolded. This excitement see
ms to have been fully justified, as the average rate of 65 per hour
during 1966-1975 suddenly jumped to over 90 per hour during
1976-1983---with the high being 187 in the latter year. Although meteor
observers seemed content with their observations of the enhanced
activity from Swift-Tuttle, comet observers were less enthusiastic as
the comet was never recovered.

Since the 1983 peak, hourly rates for the Perseids declined. With a
full moon occurring just a day before maximum in 1984, the Dutch Meteor
Society still reported unexpectedly high rates of 60 meteors per hour.
In 1985, reported rates generally fell betwe
en 40 and 60 meteors per hour in dark skies, and results were generally
the same in 1986.

As the 1990s dawned, Marsden published a new prediction. If
P/Swift-Tuttle was actually the same comet seen by Kegler in 1737, then
the comet might pass perihelion during December 1992. The comet was
recovered late in the summer of 1992. Although not one of the most
spectacular apparitions, the comet was well observed. But meteor
observers were waiting for the Perseid display of 1993. Predictions
indicated Europe was the place to be during the Perseid maximum of
1993. Observers from around the world flock
ed into central Europe and were met with hourly rates of 200 to 500.
High rates were still present during 1994, this time with the peak
occurring over the United States.

From the 1860s onward, studies of the Perseids began to include more
than just hourly rates. Numerous observers began to plot the paths of
meteors onto star charts to derive the points from which the meteors
seemed to be radiating. The most prolific obser
ver of this stream was William F. Denning, who, between 1869 and 1898,
observed 2409 Perseids and became the first person to derive a daily
ephemeris of the radiant's movement. In 1901, he published his most
precise radiant ephemeris as follows:

Perseid Radiant Ephemeris

Date

RA (deg)

DECL (deg)

July 27

27.1

+53.2

July 29

29.3

+53.8

July 31

31.6

+54.4

Aug. 2

33.9

+55.0

Aug. 4

36.4

+55.5

Aug. 6

38.9

+56.0

Aug. 8

41.5

+56.5

Aug. 10

44.3

+56.9

Aug. 12

47.1

+57.3

Aug. 14

50.0

+57.7

Aug. 16

52.9

+58.0

[A recent plotting of 102 precise photographic meteor orbits by the
author supports the general accuracy of the above ephemeris with the
daily motion of the radiant being computed as RA=+1.40 deg, DECL=+0.25
deg]

In addition to this main radiant near Eta Persei, there have been
indications that several secondary showers are also active. Minor
activity near the main Perseid radiant has been noted on several
occasions up to the present time and may have been noted a
s long ago as 1879, when Denning pointed out that he had "detected the
existence of two other simultaneous showers from Chi and Gamma Persei."
This latter shower is one of the most active of the secondary radiants
and seems to have been frequent
ly observed during the twentieth century---especially with telescopic
aid. The following observations represent some of the details.

In 1921, Ernst Opik (Dorpat) observed the Perseids telescopically
on August 10 and 12. On the latter date, he noted that 9 meteors
emanated from an oval area 5.7 degx2.2 deg across centered at RA=40.0
deg DECL=+55.6 deg.

On August 11, 1921, C. P. Adamson (Wimborne, Dorset) seemed to have
detected both the normal radiant and this southern component visually
by claiming the Perseids emanated from an elongated area extending from
RA=43 deg, DECL=+57 deg to RA=49 deg, DEC
L=+58 deg.

On August 10, 1931, C. B. Ford and B. C. Darling found a telescopic radiant at RA=40.9 deg, DECL=+54.4 deg.

On August 8, 1932, Öpik found a radiant at RA=39 deg, DECL=+54 deg.

During August 12-13, 1934, Ford found a telescopic radiant at RA=43.1 deg, DECL=+55.2 deg.

One of the most recent examples of the complexity of the Perseid
meteor shower was revealed in three studies of the radiant conducted
during 1969 to 1971, by observers in the Crimea. In addition to the
main radiant near Eta Persei, they confirmed the exis
tence of the major radiants near Chi and Gamma Persei, as well as minor
radiants near Alpha and Beta Persei. These meteor showers are generally
short-lived and possess radiants that move nearly parallel to the main
radiant. The following are summaries of the most consistent of the
secondary Perseid radiants.,

The Gamma Perseids mainly occur during August 11 to 16 from an
average radiant of RA=41 deg, DECL=+55 deg. The radiant diameter
averages about 2 deg. Rates rise and decline with those of main
radiant.

The Chi Perseids occur during August 7 to 16 from an average
radiant of RA=35 deg, DECL=+56 deg. The radiant diameter is about 2
deg. Maximum seems to occur between the 9th and 11th.

The Alpha Perseids occur during August 7 to 24 from an average
radiant of RA=51 deg, DECL=+50 deg. The radiant diameter averages about
1.5 deg. Maximum seems to occur somewhere between the 12th and 17th.

The Beta Perseids occur during August 12 to 18 from an average
radiant of RA=47 deg, DECL=+40 deg. The radiant diameter averages about
1 deg. Rates are irregular. Weakest branch of the Perseid cluster.

These secondary centers of activity have been predominantly visual
displays; however, time was taken to seek out some of these other
radiants during the Jodrell Bank radio-echo survey of the 1950's. Only
the Alpha Perseids were noted with confidence. Detected in both 1951
and 1953, the radiant was very diffuse and 8 deg in diameter centered
at RA=54 deg, DECL=+48 deg. It was detected between August 8 and 11,
and the highest radio-echo rate reached 37 per hour (the main Perseid
radiant reached radio-ec
ho rates of 50 per hour during the same years).

Other studies conducted by amateur and professional astronomers
during the last 30 to 40 years have involved specific details of shower
members. One especially interesting statistic that has been brought
forward was the trend that the Perseids seem to be brighter before the
date of maximum than afterward. In 1953, A. Hruska (Czechoslovakia)
found the average magnitude to be about 2.5 during August 8 to 12.
However, on August 12/13 it had dropped to 2.8 and by August 14/15 it
had fallen to 3.4. In 1956, Z
denek Ceplecha also showed a similar, though less pronounced decline in
brightness. During August 4 to 10, the average Perseid was near
magnitude 2.68, while during August 10 to 15 it was 2.94. The extremes
came on August 6/7 (magnitude 2.31) and August 1
3/14 (magnitude 3.18). Just as Hruska and Ceplecha's studies show
conflicting patterns representing the decline in the Perseid magnitude
distribution during August, two very recent studies seem to support
both views.

During 1983, members of the Spanish astronomical group Agrupacion
Astronomica Albireo, under the direction of Eduardo Martinez Moya,
obtained an excellent series of Perseid magnitude observations, which
seemed to support Hruska's study. Between August 1 a
nd 13, 1983, the average daily magnitude varied from 1.75 to 2.04.
Thereafter, it dropped to 2.19 by the 14th, 2.52 by the 15th, 2.77 by
the 17th, 2.92 by the 19th and 3.45 by the 20th. Robert Mackenzie
(director of the British Meteor Society) claims the magnitude
distribution of the Perseids "gives an indication of the particle mass
variation in the cross-section of the stream encountered by the Earth."
This variation seems to support Hruska's study.

Another excellent series of magnitude estimates were made by Paul
Roggemans (Brussels, Belgium) during July 27 to August 16, 1986.
Observing in darker skies than the Spanish group, Roggemans detected
1315 Perseids and gave the average magnitude of the sho
wer as 3.10. Roggemans' estimates were very consistent throughout the
shower's duration with variations being typically less than 10% on any
given day. However, there were two exceptions. The first came on August
5/6 and 6/7, when the average magnitude dr
opped to a low of 3.54. The second drop occurred on August 9/10 and
10/11 when the average magnitude reached 3.71. This set of observations
seems to support Ceplecha's study.

All of the above magnitude studies (and many more not discussed
here) seem to have one thing in common---they point to an irregular
mass distribution within the Perseid stream. Filamentary structure
seems the best explanation. During some years, the filam
ents are encountered in rapid succession by Earth's passage through the
Perseid stream, thus accounting for the consistent magnitude estimates
followed by a steady decline. In other years, the filaments are spread
out across the stream's width, thus causi
ng the consistent average magnitude estimates to be disrupted by
periods of activity from primarily brighter or fainter meteors.

Another statistic that has been brought forward during the last 30
to 40 years has been the percentage of Perseids that possess persistent
trains. This is a major factor long noted in the separation of Perseids
from other active showers occurring during t
he first half of August. Miroslav Plavec used the records made at the
Skalnate Pleso Observatory (Czechoslovakia) to produce one of the most
ambitious studies of train phenomena to date. He studied 8,028 meteors
observed between 1933 and 1947, and found t
he following percentages: 45% possessed trains in 1933, 60% in 1936,
35% in 1945 and 53.5% in 1947. The variations could not be correlated
to sunspot numbers. Taking an average of meteor train activity noted in
various publications between 1931 and 1985, the author has found the
average value to be 45% for nearly 60,000 meteors.

Orbit

More orbits have been computed for the Perseids than for any other
meteor stream, with the first coming during the early 1860's. During
the last few decades photographic and radio-echo techniques have
enabled the first precise orbital determinations. The Author has
accumulated 102 precise photographic meteor orbits from the various
United States, Soviet Union and Czechoslovakian lists. The following
orbit was revealed.

AOP

AN

i

q

e

a

149.2

139.5

113.2

0.942

0.902

9.641

Two major radar surveys revealed the Perseids during the 1960's.
B. L. Kashcheyev and V. N. Lebedinets found the Perseids during a 1960
survey at the Kharkov Polytechnical Institute (BL1967), while Zdenek
Sekanina determined an orbit from data gathered du
ring the 1969 session of the Radio Meteor Project (S1976).